The nation’s largest employer and retailer has filed a complaint with the National Labor Relations Board — its first in a decade — seeking to prevent a group known as OUR Walmart and supported by the union from holding at hundreds of stores what the group says will be the biggest protests of this kind against the company.

Labor experts caution that the complaint, filed Thursday, could be a warning shot to discourage workers from participating, since the labor board often takes months to rule, but it nonetheless reflects how seriously the company has come to view a group it once dismissed as a nuisance.

William B. Gould IV, a Stanford University law professor and chairman of the NLRB under President Bill Clinton, said the protests were more about employment conditions and retaliation against employees than a unionization drive.

‘‘I don’t see this translating into a great deal of success in terms of unionizing Walmart or in terms of being particularly effective in improving conditions,’’ Gould said. ‘‘But I must say if they’ve gone to the NLRB on this, that must show that Walmart is really concerned.’’

Since October, OUR Walmart has staged smaller strikes at individual stores, though none of those disrupted Walmart’s operations, the company said. OUR Walmart said that last month, 88 workers went on strike at 28 stores in 12 states.

OUR Walmart has also flown dozens of workers to company headquarters and has encouraged churches to hold a ‘‘Black Friday Prayer Vigil’’ to object to the company’s treatment of workers on the planned protest day.

Walmart had initially brushed off these actions as inconsequential public relations efforts. But it is taking them more and more seriously, sending a memo advising managers how to deal legally with protesters and warning some union-friendly groups they might face arrest if they trespass during the protests.

All this points to an increasingly fierce contest between Walmart and labor groups that are bent on mobilizing and organizing the company’s workforce, with a near-term goal of pressing for higher wages and a longer-term goal of emboldening workers to demand a union.

‘‘You are going to see unprecedented activity from now and going into Black Friday,’’ said Dan Schlademan, a principal organizer of the events and director of Making Change at Walmart, an affiliate of the United Food and Commercial Workers Union.

That union has made Walmart a target because the company has helped put many unionized supermarkets out of business and helped push down wages at many competitors. Walmart, moreover, has vigorously resisted unionization drives.

In this week’s planned events, OUR Walmart, which stands for Organization United for Respect at Walmart, is enlisting a broad range of allies, arranging fliers and letters that community, church, and civil rights groups can use to publicize the Black Friday protest.